Wireless systems enable a mobile user to communicate with others through the broadcast of electromagnetic radiation. Wireless terminals generally include receiver circuitry for capturing broadcast signals. Many receivers include amplification circuitry at the front-end to amplify received signals before down converting for further processing. Such amplification circuitry will amplify noise mixed in with the received radio frequency (RF) signal and can generate its own noise as well. The generation of noise can reduce the quality of the received signal, and consequently, reduce the distance of signal propagation over which information in the signal may be detected. Conversely, reducing the receiver's noise figure (NF) can increase the ability to detect wireless broadcast signals. Thus, reducing the NF of a receiver may enable detection of information in signals transmitted at increasing greater distances from the receiver.
RF amplification circuits are generally designed to process signals within a specified frequency range. A conventional television broadcast systems, which is an example of a wide-band system operating from about 50 MHz to about 860 MHz, requires a receiver having a certain minimum amplifier gain, return loss and NF in order to efficiently convert a broadcast signal for viewing. Amplifier gain quantifies the magnitude of the amplification provided by an amplifier. Return loss quantifies the degree to which the amplifier's input impedance matches the impedance of a preceding circuit (i.e., a source impedance). The NF quantifies the amount of additive noise introduced by the amplifier. An increasingly higher gain indicates the amplifier provides increasingly larger signal amplification, an increasingly higher return loss indicates the input impedance of the amplifier is increasingly better matched to the source impedance. A decreasing or lower NF indicates the amplifier introduces an increasingly lower level of noise.
The dynamic gain range of an amplifier is another parameter that can affect receiver performance. Since atmospheric conditions and distances between a transmission station and a receiver can vary, so can signal strength. A receiver must generally be able to process RF signals over wide dynamic gain range to accommodate variations in signal strengths. A useful RF signal may range from a low of about −90 dBm to a high of about −25 dBm at the receiver. Such a wide range can be accommodated by adjusting receiver gain, for example, by setting a lower gain when the received signal strength is greater and setting a higher gain when the received signal strength is weaker. If the received signal is strong, the noise introduced by an amplifier is less significant and a larger NF can often be tolerated. If, however, the received signal is weak, the noise introduced by an amplifier can be significant, and therefore, a high NF is no longer acceptable. In addition to gain and NF parameters, matching the receiver's input impedance to the signal's source impedance is also an important consideration. A higher return loss generally means the receiver's impendence matches better with the signal's source impedance and thus more useful signal is available for processing. What is needed then is a wide-band frequency amplifier with a widely adjustable gain that allows for good impedance matching to a source circuit, and has a low NF.